This invention concerns a production method by permeation of a gaseous mixture containing a permeable component, of at least two gaseous fluxes enriched with said component as compared to the gaseous mixture, the respective contents of these at least two fluxes enriched with said component being substantially different. Advantageously one of the enriched fluxes is hydrogen, that can be used, for instance in chemical reactors.
In the following description, the terms xe2x80x9csubstantially different contentsxe2x80x9d will mean:
contents in the most permeable component which differ by at least 4 or 5 points, in other words for example, where the xe2x80x9cless enriched flux/more enriched fluxxe2x80x9d pairs are about 70%/90%, 85%/95%, or 95%/99%;
and/or contents of at least one impurity are in a ratio of at least 2 or 3, in other words for example, for one impurity, the xe2x80x9cimpurity contents of the less enriched flux/impurity content of the more enriched fluxxe2x80x9d pairs are approximately 3%/1%, 1%/0.2% or 1000 ppm/100 ppm.
This invention is applicable to the production of such fluxes from gaseous mixtures produced from a chemical unit where one or several components can be recovered and/or recycled to form new reagents for use in this chemical unit. For example, in ammonia synthesis, the methods of the type described above are suitable for the production of hydrogen-enriched fluxes and recycled in the ammonia synthesis unit, as well as the production of a flux of very pure hydrogen used in another process.
The invention can also be used with refinery gas that contains, for example, between 30 and 85% of hydrogen moles. This invention permits to produce with a good global extraction yield both a highly enriched hydrogen flux, as compared to the initial mixture, as well as one or two other mediumly enriched fluxes.
The production of a flux highly enriched in the most permeable component is well known, for example from the publication xe2x80x9cPressure Swing Absorptionxe2x80x9d by Ruthven et al, paragraph 8.4. However the production of such a flux with a sufficient extraction yield requires internal recirculation that is very costly in investment and energy, as confirmed by the authors. Without these internal recirculations, it is possible to obtain a pure permeate, but with an extraction yield so low that the method is generally too expensive.
Another method is known, for example from the documents U.S. Pat. No. 4,180,388 and EP-A-0-700 709, where the gaseous mixture is subject to arrangements of permeators in series on the waste from the previous permeator, the permeate of each permeator forming a hydrogen-enriched flux with a different content.
However these arrangements or layouts do not permit the acquisition of enriched fluxes with a very high content. In the document U.S. Pat. No. 4,180,388, the maximum enriched hydrogen content is that produced by the first permeator fed with the gaseous mixture, and does not exceed the hydrogen count obtained from the second permeator by more than slightly over two points.
The purpose of this invention is to propose a separation method by permeation that is easy to set up and that permits to obtain at least two enriched fluxes, one of which has a very high purity level, while maintaining a high global extraction yield.
For this purpose, the invention aims at furnishing a method in which a first permeation unit is supplied by the gaseous mixture, a second permeation unit is supplied by the non-permeate from the first permeation unit, the permeate produced by the second unit forming a first enriched flux, and a third permeation unit is supplied by the permeate from the first permeation unit, the permeate produced by the third permeation unit forming a second enriched flux with a higher component content than the first flux.
According to other features of this method, either used together, or according to all the combinations technically possible:
the flow of the second enriched flux can be adjusted;
the content of said component in the second enriched flux can be adjusted according to the pressure on the non-permeate side of the third permeation unit;
the non-permeate of the third permeation unit forms a third flux with a weaker component content than that of the second flux.
This invention is also aimed at furnishing a method of supplying a processing unit with at least one gaseous flux rich in one permeable component, wherein at least two enriched fluxes can be produced using a method such as that described above, the processing unit being supplied by at least one of the first and third enriched fluxes.